1. Field of the Invention
The present invention is directed to a system for blending a graphic with another image or video.
2. Description of the Related Art
Television broadcasts of events can be enhanced by providing graphics that are blended with other images or video to convey information to viewers. Such information can be related to the event, people involved in the event, or any other subject that the broadcaster desires. For example, during the broadcast of a football game a graphic can be employed to indicate the field location to which an offensive team must advance in order to obtain a first down. Logos and other graphics can also be blended with a video of an event. In some instances, such as adding a first down line or an advertisement to an image of a playing field, it is desired to blend the graphic with the video such that the graphic appears to be part of the live event, and not superimposed on the video. For example, it may be desired to add a line or logo to a video presentation of a sporting event such that the line or logo appears to be painted on the field.
When blending graphics with a video, broadcasters may want to vary the degree to which the graphic is blended. For example, when blending graphics to appear to be part of the event, it may be desired to vary the amount of blending to account for occlusions. That is, if a person or object is positioned in front of the location where the graphic is to be added, the graphic would look more realistic if the graphic is not drawn over the person or object. A broadcaster may also want to add a graphic to a video representation of a scene such that the graphic shows some of the texture of the underlying scene. A broadcaster may also wish to vary the blending of an image to show hard edges or soft edges. In order to accomplish these results, a blending coefficient can be generated for each pixel in the graphic to indicate the degree of blending between the graphic and the program video. One example of such a blending coefficient is an alpha signal that is used with a keyer for blending a graphic.
Most attempts to generate blending coefficients have utilized chroma-key techniques. Chroma-keying usually entails choosing a color in a video signal to be replaced with a graphic. In most instances, use of the color in the video is controlled and the blending is limited to either replacing or not replacing, without degrees of blending. On the other hand, a broadcast of a live event such as a sporting event may include many different colors and lighting conditions, different hue and brightness conditions, and different color balances that are not controlled by the broadcaster. During the broadcast of a football game, the broadcaster has no control over the color and lighting of either the field or players' uniforms. In addition, changing weather patterns can also affect the appearance of certain colors. For example, a broadcaster may wish to blend a graphic to a video of a sporting event such that the graphic appears to be painted on the field. In such a case, a system should be able to make a distinction between a "green" of a grass field, where blending is to occur, and a "green" of a player's uniform, where no blending is to occur. The standard chroma-key techniques cannot perform all of the above functionality in real time, and cannot make all the necessary fine distinctions between the colors appearing at a live event.
Accordingly, a system is needed for generating a blending coefficient that provides a broadcaster with flexibility in blending a graphic to the video presentation of a live event.